Dislocation and Strong Ground Motion Zoning Under Scenario Faults for Lifelines
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13th World Conference on Earthquake Engineering Vancouver, B.C., Canada August 1-6, 2004 Paper No. 125 DISLOCATION AND STRONG GROUND MOTION ZONING UNDER SCENARIO FAULTS FOR LIFELINES Shiro TAKADA1, Yasuko KUWATA2, Abbas MAHADAVIAN3, Resa RASTI4 and Tsutomu IMAI5 SUMMARY Present paper proposes a seismic hazard map in Tehran City to evaluate the seismic safety of gas supply systems extended whole in the city. First, the seismo-tectonic feature in Tehran area is investigated and scenario active faults that cause severe earthquakes are specified giving fault parameters. Based on the fault analysis, synthetic strong ground motions are estimated on the surface ground by using convolution method for element earthquakes and non-linear amplification factors. Then surface fault dislocation is estimated by using elastic dislocation theory. Furthermore, the potential of liquefaction and land slide is involved in the hazard map. INTRODUCTION According to the zoning maps of seismic hazard prepared for Iran (Iran Code 2800), Teheran is located in a region with relatively high seismic hazard. Therefore, it is of special importance to identify the seismogenic factors of the region. Then, strong ground motion parameters were estimated along the gas pipeline and pressure release stations for making them resistant to earthquake. Complementary seismotectonic studies were performed including the re-study of satellite images and aerial photographs on the near site area with a radius of 100 kilometers. In some cases, the faults near the new trenches of expressways and foundation excavations of the buildings under construction were inspected. Finally, the locations of faults in and around Teheran were exhibited on the map of Teheran. The seismotectonic profile of the near faults was prepared and their situations were determined. The city of Teheran is located in the boundary of the two seismotectonic provinces, i.e., Alborz and Central Iran. This boundary is not clear cut. On the other hand, the epicentral errors of the past earthquakes are more than tens of kilometers. Therefore, seismic data were collected in a radial way. 1 Professor, Kobe University, Kobe, Japan. Email: [email protected] 2 Assistant professor, Kobe University, Kobe, Japan. 3 Assistant professor, Power and Water Institute of Technology, Tehran, Iran. 4 Lecturer, Power and Water Institute of Technology, Tehran, Iran. 5 Engineer, Mitsubishi Heavy Industries, Ltd., Hiroshima, Japan Strong ground motion and surface ground dislocation due to fault ruptures are estimated by introducing fault rupture analysis and elastic dislocation theory. Also, ground failure of liquefaction and land slide potential are estimated along the gas network and stations. FAULTS AND STRONG GROUND MOTIONS Major Faults and Parameters Table 1 listed the major active faults near Tehran. As an example, outline of the North Tehran Fault and relevant historical earthquakes is shown below. The North Tehran Fault is 90 km long and located on the north of Teheran. It has E-W to ENE-WSW strike and has thrust mechanism. This fault has been traced in the North Tehran mountains from the east of Lashgarak (Saboo Village) in the northeast of Tehran to KazemAbad locality (2 km east of Kalak and north of Tehran-Karaj Expressway) and in the city of Karaj in the west. In most places including north of Tehran and at the foothills, this quaternary alluvia has caused the Eocene Karaj Formation (Alborz Border Folds) to be thrust over Hezardarreh and NTF (pediment zone of Central Iran). The sudden change in height between the city of Tehran (with an average elevation of 1300 masl) and its highest peak (Tochal, 3933 masl) in a distance of less than 10km is a significant topographic characteristic of Tehran for which North Tehran thrust movements are responsible (Tchalenko et al. [1]). The thrust dip of this fault is highly variable: 10-45 degrees towards north on the west of Kan, 27-40 degrees towards north on the east of Kan, 70-80 degrees towards NW at Farahzad, 40 degrees towards NNW on the west of Lashgarak Valley and about 30 degrees towards north on the NE of Saboo Village. It can be assumed anyway that the dip of NTF is milder than 75 degrees, because this fault Table 1 Characteristics of some important faults at the project area Approximate. Max attributed Fault names Mechanism General trend Length (Km) magnitude (M) North Tehran 90 Thrust E-W 7.3 Niyavaran 18 Thrust with left lateral ENE-WSW 6.5 strike-slip component Mahmoodiyeh 11 Thrust E-W 6.2 Davoodiyeh 4.5 Thrust E-W 5.7 South Mehrabad 10 Thrust NE-SW 6.2 North Ray 17 Thrust E-W 6.5 South Ray >18 Thrust ENE-WSW >6.5 Kahrizak >40 Thrust E-W 6.9 Parchin 73 Reverse NW-SE 7.2 Qasr Feeroozeh 18 Reverse NW-SE 6.5 Shiyan Kowsar 15 Thrust NW-SE 6.4 Upper Telo 10 Thrust NW-SE 6.2 Lower Telo 20 Thrust with right lateral NW-SE 6.5 strike-slip component Latyan 11 Reverse WNW-ESE 6.2 Baghfeyz 4.5 Thrust with right lateral NW-SE 5.7 strike-slip component Sorkhesar 22 ThrustE-W to 6.6 WNW-ESE Hamsin 9 ThrustE-W to 6.1 WNW-ESE Bibishahrbanoo 5 Thrust WNW-ESE 5.8 is a branch of Mosha Fault. Hence, a milder dip than that of Mosha Fault should be considered for it to reach Mosha Fault in depth. This is a seismogenic quaternary alluvia considered as a branch of Mosha Fault. Contrary to Mosha Fault, this quaternary alluvia does not have a distinct fault scarp (Berberian and Yeats [2]). Due to the scarcity of data, its seismic history is not clearly known, but the following earthquakes have been probably caused by this fault (Berberian et al. [3]): - The earthquake of Feb 23, 958 B.C., with an estimated Ms 7.7(ISC) and 7.4 (Ber) - The earthquake of May 1177 between Shahr Ray and Qazvin, estimated Ms 7.2 - The earthquake of December 24, 1895, Tehran - The epicenter of the earthquake of Roodbar Qasran, M4.1, has been determined 25 and 35 km north of North Tehran thrust in the north of the city of Tehran, and this earthquake has probably resulted from the movement of this fault, but there is no strong reason to support this idea. - The earthquake of Najjarkola NE of Tehran, Oct 26, 1989 As for the earthquakes of 855-856 (exact date not known) and 1177, the responsible quaternary alluvia or faults cannot be determined, but could have caused the rupture of NTF. After the close discussion about faults near Tehran, we could reach to following results with respect to earthquakes caused active faults. We use two approaches to estimate strong ground motion and acceleration at the seismic base: probabilistic and deterministic approaches. For the latter approach, we assumed 4 scenario earthquakes as shown in Table 2, which lists fault parameters to calculate strong ground motions. The sketch of scenario faults for both approaches is shown in Fig.1. Table 2 Scenario faults in the project L W Small Moment D τ Θ δλ Upper Case Fault n depth of (km) (km) M M (m) (sec) (°) (°) (°) fault (km) Case1 North Ray 17 9 5 6.5 0.63 1.57 277 75 90 5 5 South Case2 20 10 5 6.6 0.7 1.85 277 75 90 6 5 Ray North Case3 30 30 5.3 7.2 1.41 2.78 311 75 90 9 0 Tehran Case4 Mosha 20 22 5.3 7.1 1.25 2.03 309 75 90 8 0 Mosha North Tehran North Ray South Ray ParchinKahrizak ScenarioLevel2 earthquake HistoricalLevel1 earthquake KahrizakParchin Fig. 1 Faults for scenario earthquake Statistical Ground Motions In the probabilistic approach, peak ground acceleration is calculated based on the statistical analysis of available earthquake catalogue entries for a certain return period. In this method, at first the probabilistic event of earthquake with magnitude M at distance R is calculated. The seismic hazard will be obtained based on area and line source with random variable under the Poisson process. Under the assumption of the Poisson process, the earthquake occurrence probability during the period ∆T , in which the intensity Y is over y , is given by; P(ν ;∆T ) =1− exp(− p(Y ≥ y) ⋅ ∆T ) (1) Fig. 2 shows the exceedance probability of ground motion for return periods. Table 3 lists the fault parameters for statistical ground motions. Fig. 3 shows the procedure to simulate earthquake ground motion. First, a time history of ground motion is simulated for a given fault and then an enveloped velocity response spectrum is determined in order to cover relevant velocity response spectra. Finally, a sample time history for the statistical ground motions is obtained. Max acc. (gal) 1 0 100 200 300 400 0.1 T ⊿ 0.01 10 yrs年 3030 yrs年 EQ. occurrenceEQ. probability during 50 y年rs 100100 y年rs 0.001 Fig. 2 Earthquake occurrence probability for given return periods Table 3 Fault parameters for statistical ground motions L W Small Moment D τθδλ Upper Case Fault n depth of (km) (km) M M (m) (sec) (°) (°) (°) fault(km) Case8 Parchin 73 28 5.3 7.2 1.41 6.76 250 75 0 9 5 Case9 Parchin 73 28 5.3 7.2 1.41 6.76 250 75 90 9 5 Case10 Parchin 73 28 5.3 7.2 1.41 6.76 250 75 180 9 5 Case11 Parchin 73 28 5.3 7.2 1.41 6.76 250 75 270 9 5 Case12 Kahrizak 50 20 5.2 6.9 0.99 4.63 260 75 0 7 5 Case13 Kahrizak 50 20 5.2 6.9 0.99 4.63 260 75 90 7 5 Case14 Kahrizak 50 20 5.2 6.9 0.99 4.63 260 75 180 7 5 Case15 Kahrizak 50 20 5.2 6.9 0.99 4.63 260 75 270 7 5 100 100 50 0 -50 10 Acc.